In petrochemical and refinery processes, knowledge of the composition of process streams is required when evaluating, for example, product purity or the efficiency of operation of a particular part of a process, such as a reactor or a distillation column. Compositional information may be obtained on-line, wherein a sample of the contents of a process stream or vessel, for example, is analysed without the need for any manual handling and transfer of the sample from the process to a remotely located laboratory. On-line analysis can therefore reduce the time required to obtain the desired information, while also reducing the chances of sample contamination which may occur when the sample is taken and transferred to the point of analysis. Known on-line techniques for obtaining compositional information of a process stream are exemplified by on-line gas chromatography, and near infrared (NIR) spectroscopy. For example, U.S. Pat. No. 6,820,013 describes the use of NIR spectroscopy to quantify the concentrations of the components in a mixture of solvents; EP-A-0 859 236 describes the use of NIR spectroscopy to obtain compositional information of crude oil being fed to a crude distillation column, and US 2002/0067481 describes a transmission cell that may be attached directly to a conduit for spectroscopically analysing the contents.
NIR spectroscopic measurements may be performed on substances in either the liquid or the gas phase. However, NIR spectra of liquids comprising strongly interacting components, particularly components that interact through hydrogen bonding, can suffer from poor resolution due to broadening of the NIR absorption bands, which can limit the accuracy of any parameters calculated therefrom.
An on-line mass-spectrometer has been previously described in The Analyst, 1998, vol 123, pp 343-348, wherein the vapourisation of a liquid sample for feeding to a mass spectrometer using a heated capillary tube and a feed of helium carrier gas is described. Additionally, RU 336563 describes an apparatus for creating micro-concentrations of vapourised liquids in a carrier gas by means of a liquid-filled capillary tube fitted with a wick.
There remains a need for an improved means of obtaining an on-line gas-phase NIR spectrum of a liquid sample which avoids the need for a separate carrier gas supply.